Minimum number of observations in window required to have a value
(otherwise result is NA).

freq : string or DateOffset object, optional (default None)

Frequency to conform the data to before computing the statistic. Specified
as a frequency string or DateOffset object.

pairwise : bool, default False

If False then only matching columns between arg1 and arg2 will be used and
the output will be a DataFrame.
If True then all pairwise combinations will be calculated and the output
will be a Panel in the case of DataFrame inputs. In the case of missing
elements, only complete pairwise observations will be used.

ddof : int, default 1

Delta Degrees of Freedom. The divisor used in calculations
is N-ddof, where N represents the number of elements.

Returns:

y : type depends on inputs

DataFrame / DataFrame -> DataFrame (matches on columns) or Panel (pairwise)
DataFrame / Series -> Computes result for each column
Series / Series -> Series